I. FIELD OF THE INVENTION
This invention relates to a process for producing silicon nitride.
II. DESCRIPTION OF THE PRIOR ART
Silicon nitride is a ceramic material that is in increasing demand these days for a variety of uses. For example, in the form of submicronic powders it is used for the production of sintered components. In the form of whiskers, it is used as a reinforcement for high temperature materials.
High purity Si.sub.3 N.sub.4 is generally produced by a gas phase reaction of silicon halide and ammonia. Another method involves the carbo-nitridation of SiO.sub.2 in which fine amorphous silica is reacted with lampblack at temperatures below 1500.degree. C. in a nitrogen atmosphere to produce Si.sub.3 N.sub.4. However, in the latter method, about 3-4% by weight of unreacted SiO.sub.2 remains in the nitride product.
An improvement of the carbo-nitridation method is disclosed in Japanese patent application publications Nos. 1981-17910 and 1980-15946 both in the name of Toray Industries Inc. and laid open for inspection on Feb. 20, 1981 and Feb. 4, 1980, respectively. In the methods of these patent publications, extremely fine silica powder is dispersed in an organic polymer of the polyacrylonitrile group and is heat treated in a nitrogen or ammonia atmosphere. Despite statements in the patent publications that these methods result in the formation of a highly pure, finely divided silicon nitride product, the present inventors have not been able to obtain a pure product in this way. In particular, the product tends to be contaminated with silicon carbide formed as a by-product during the reaction and there is incomplete conversion of the SiO.sub.2.
An object of the invention is therefore to provide an improved method of preparing silicon nitride of high purity.